The invention concerns double-resonance generators, intended in particular for but not limited to powering an X-ray tube.
An X-ray tube mounted, for example, in a medical radiography apparatus, comprises a cathode and an anode, both contained within a vacuum-tight casing, so as to form an electric insulation between those two electrodes. The cathode produces an electron beam which is received by the anode on a small surface constituting a focal area from which the X-rays are emitted.
On the application of a high voltage by a generator to the terminals of the cathode and anode, a so-called anode current is established in the circuit through the generator producing the high voltage. The anode current crosses the space between the cathode and the anode in the form of the electron beam which bombards the focal area.
The characteristics of the X-rays emitted by the tube, notably, their hardness, depend on numerous parameters, among which is the high voltage value applied to the electrodes. The high voltage has to be adjustable in order to obtain the characteristics sought and must remain constant throughout the time of radiological exposure, in order not to modify the operating characteristics of an X-ray receiver which receives the X-rays having crossed the object under examination.
The X-ray tubes for medical diagnosis operate by pulses. It is therefore important for the time of establishment of the high voltage as well as the time of return of that high voltage to a zero value to be as short as possible.
An X-ray tube high voltage generator comprises a thyratron inverter receiving a direct-current voltage at its terminals. This thyratron inverter is of the type containing a transistor half-bridge, each arm of the half-bridge comprising a switch containing a transistor and an antiparallel-mounted recovery diode. The signal delivered on output of the switch is filtered by a double-resonance circuit. The filtered signal is applied to the primary of a step-up transformer. The secondary of the step-up transformer is connected to a rectifying and filtering circuit comprising at least one diode half-bridge and voltage filtering capacitors.
The principle of the double-resonance converter resides in the topography of its resonant circuit. It comprises an inductance coil Lr connected in series with a capacitor Cr in parallel with an inductance coil Lp. This configuration gives the system a resonant frequency and an antiresonant frequency. The former originates from the series inductance coil Lr and capacitor Cr, and the latter originates from the parallel inductance coil Lp and capacitor Cr.
The conduction delay can be regulated and, therefore, so can the frequency of use of the transistors of each of the switches. This frequency is in relation to that of the resonant circuit defined above. The normal operating zone of the generator lies between the two natural frequencies of that circuit. The antiresonant frequency makes it possible to attain low powers and the resonant frequency makes it possible to attain high powers.
In other words, the control variable of the generator here is the delay Td for starting of the transistors from the moment of passage of the current of the thyratron inverter to zero value. One then speaks of a delay time control. More precisely, the control employs a sampling of quantities at moments defined by passage to zero of the current crossing the series inductance coil of the resonant circuit following conduction of the transistor of a switch.
It therefore appears that the power transmitted to the X-ray tube can be controlled by the operating frequency of the thyratron inverter and, consequently, by the delay for starting Td.
However, the known control circuits make it possible to establish the direct-current voltage supplying the X-ray, tube at its desired value only after too long a time, which is manifested by a loss of tinen and, by an X-ray dose unnecessarily received by the patient.
Furthermore, these control circuits leave a ripple present in the supply voltage of the tube after its establishment. The ripple is at a frequency of 100 or 300 Hz depending on the single-phase or three-phase supply tube used. These ripples are all the more disturbing as X-raying can attain thirty images per second, and that is manifested by image instability.
The present invention is therefore intended to remedy the problems mentioned above by proposing a generator control making it possible to reduce substantially the time of establishment of the direct-current voltage supplying the tube at the start of X-raying.
The invention is also intended to reduce the ripples of the continuous rated voltage supplying the tube.
An embodiment of the invention therefore proposes a method of control of a double-resonance generator delivering a direct-current output voltage supplying a load, for example, an X-ray tube, in which an output current circulates, the method involving a delay time control of the transistors of the thyratron inverter of the generator.
According to a general characteristic of the invention, the control involves a static term and a corrective term. A first table (range of identification of the generator) is elaborated, containing for predetermined couples of voltage and output current values the corresponding values for the static term. A second table is also elaborated, containing corresponding desired values for a voltage at the terminals of the resonant circuit capacitor (called xe2x80x9cfirst voltagexe2x80x9d), as well as for the current crossing the inductance coil connected in parallel on said capacitor. The second table also contains corresponding gain values. Furthermore at a current sampling time, the control calculated at the preceding sampling time is applied, and the control intended to be applied at the following sampling time is determined. In addition, the static term of the following control is determined from the first table and from the voltage and output current values estimated at the following sampling time. The corrective term of the following control is determined from the second table, from the voltage and output current values, from the first voltage and from the the inductance current, all of those values being determined at the current sampling time, as well as from the value of the corrective term calculated at the preceding sampling time.
According to an embodiment of the method, the voltage and output current values are estimated at the following sampling time from a predetermined evolution of a desired output voltage. Determination of the corrective term of the following control involves measurement of the current value of the output voltage, of the current value of the first voltage and of the current value of the inductance current. This determination also involves calculation of the sum of the products of the respective differences between those measured current values and the corresponding desired values by the associated gains, the sum being increased by the product of the preceding corrective term by the associated gain.
Determination of the corrective term of the following control further advantageously entails addition of the product of an integral corrective element relative to the output voltage by the associated gain. This advantageously makes it possible to catch model errors.
According to an embodiment of the invention, the elaboration of the first table as well as the elaboration of the desired values for the first voltage and inductance current is obtained by simulation from a generator representation circuit, in which simulation of the desired values are obtained in steady state by using only one transistor of the thyratron inverter and by inverting, at the beginning of each cycle, the variables belonging to the common part of the representation circuit. Furthermore, elaboration of the gains entails the elaboration of a dynamic model, the coefficients of which are obtained by variations of the initial conditions.